The researchers found that the mean vitamin D concentration was below the study laboratory's lower limit of normal (P < 0.0001) and did not change during the course of the disease. In patients with bulbar onset, the concentrations were higher (P = 0.003) and were negatively associated with body mass index (BMI; P = 0.0095). Vitamin D concentrations predicted worse prognosis in models with ALS Functional Rating Scale-Revised and BMI as covariates.

"Our study adds to the growing literature on the relationship between neurological disorders and hypovitaminosis D. In this study, there was a negative effect of higher vitamin D levels on the course of ALS, a finding at odds with some previous studies," the authors write. "The mechanisms underlying the relationship between vitamin D and ALS are still obscure; well-designed prospective studies are needed to better understand the involvement of vitamin D in ALS."